The present invention relates to a process for producing hydrogen fluoride from an aqueous solution of hydrogen fluoride and sulfuric acid by steam stripping at an elevated temperature in order to vaporize hydrogen fluoride of a desired concentration.
Austrian Pat. No. 236 340 discloses a process for producing hydrogen fluoride by distilling hydrogen fluoride from a mixture of ammonium fluoride and sulfuric acid in a distillation column having 15 theoretical plates. A good yield requires a temperature of 135.degree.-170.degree. C. at the base of the column. Owing to the corroding conditions, only impregnated graphite can be used in the heat exchanger; this is the only material that resists a maximum pressure of 600 kPa and a temperature of 165.degree. C. For this reason, very large and expensive heat exchangers have to be used for a good yield of hydrogen fluoride. The hydrogen fluoride return necessary for obtaining water-free hydrogen fluoride at the top of the column also requires an additional energy feed at the base of the column.
The stripping of hydrogen fluoride by an inert gas from a mixture of sulfuric acid and an aqueous solution of ammonium fluoride and ammonium bifluoride has also been suggested (U.S. Pat. No. 3,455,650). In this case, however, the inert gas flows along with the hydrogen fluoride being stripped, interferes with the condensation of the hydrogen fluoride, and must be separated from it at a subsequent stage. This results in losses of hydrogen fluoride, and a treatment of the outlet gases is required.
Also known is a two-stage process (British Pat. No. 1 060 519) in which solid ammonium fluoride - ammonium bifluoride is mixed with strong sulfuric acid at 150.degree. C., whereby most of the hydrogen fluoride is released, whereafter steam is fed into the mixture and the pressure is reduced in order to release the remaining hydrogen fluoride from the mixture. This process has disadvantages: firstly, the aqueous solution of ammonium fluoride and ammonium bifluoride obtained from the ammoniation of fluorosilicic acid, H.sub.2 SiF.sub.6, must be evaporated dry in order to produce solid ammonium fluoride - ammonium bifluoride, and secondly, a reduced pressure must be used at the vaporization stage.
The object of the present invention is to provide a less complicated and more economical process for the production of hydrogen fluoride from an aqueous solution of hydrogen fluoride and sulfuric acid by distillation, in order to produce hydrogen fluoride of a desired concentration. By the process according to the invention, it is possible to separate the hydrogen fluoride to such a high degree that its content in the H.sub.2 SO.sub.4 -bearing residual solution is less than 0.1% HF.